Impact printers utilize a plurality of print hammers or hammersprings arranged in a hammerbank. The print hammers or hammersprings are held before release by means of permanent magnetism.
The individual print hammers are formed of a single piece of steel plate which is ground and electro-discharge machined into a spring member or hammerspring having preferably a relatively thin tapered neck capped by a head. Each print hammer or hammerspring has a tip, pin or wire at the head end for impacting a ribbon. The ribbon impacting is then received as a printed dot on paper that is to be printed upon and which is supported by a platen.
The upper part or head of the print hammer or hammerspring is held in a retracted position by a permanent magnetic force against a pole piece until released. When the permanent magnetic force is overcome or nullified by current flow or electrical discharge through electrical coils, the print hammer is released. This causes the tip, pin or wire of the head of the print hammer or hammerspring to forcibly and rapidly contact the ribbon to effectuate a printing against the ribbon onto the paper. Immediately thereafter, the print hammer is captured again and held by the permanent magnetic force.
The print hammer or hammerspring acts like a spring by flexing along its neck. When held by the magnetic force, the print hammer is held under a bending moment or tension. Desirably, the print hammer is made of a high strength alloy steel, which can be placed under high tension to give the high energy at the time of release, which generates higher printing energy. At the same time the material must have a high magnetic saturation to secure the hammer magnetically against the pole pieces. Higher saturation induction of the hammerspring steel allows higher flux carrying capacity. This could effectively reduce the volume of the steel in the area contacting the pole pieces and increase the speed of moving, which results in a higher speed of printing. Another desirable quality is strength and toughness of the steel, so that the hammerspring or print hammer will have a life consonant with that of the printer.
While high purity iron produces a very high magnetic saturation (21.8 kilogauss (KG)), it lacks the mechanical strength needed. Alloy steels after proper heat treatment, depending upon the grade used, have acceptable mechanical and fatigue strengths, but lack adequate direct current (DC) magnetic saturation for the design optimizations.
Currently available ultra high strength steels, such as 300M, 4340 and tool steels, all have high carbon (C) concentrations that cause inferior magnetic properties. In addition, the usage of carbon as the main hardening element increases the formations of M.sub.3 C, M.sub.6 C and M.sub.23 C.sub.6 (M=Fe, Cr, Mo, W, V and etc.), and stabilizes the lath form of martensite structure during the conventional water or oil quenching operations to increase both tensile and yield strengths.
At the same time, the presence of the high concentration of carbon in the steel reduces magnetic saturation and permeability of the steel. In certain instances, it also reduces the toughness and fatigue resistance. In contrast to the conventional quench-and-harden high strength steels, there are several types of steel, which have exceptional high fatigue resistance that are hardened without utilizing the conventional quench hardening process. Instead, these steel alloys use the inert gas or air as the quench media and use the precipitation hardening process as the strengthening mechanisms. These alloys are highly desirable for hammerspring applications since they have much better dimensional stability and contain very little quench stress.
The secondary hardening martensitic steels are hardened by carbide precipitation mechanisms that require considerable amounts of nickel and cobalt. They require a solution heat treatment that is conducted at about 1600.degree. F. and then air-cooled to produce martensitic structure. After solution heat treating, these steels are subjected to a precipitation hardened process which is conducted at around 950.degree. F. to produce tempered lath martensite and to achieve the optimal mechanical properties.
The resulting final microstructure has good resistance to the dislocation recovery even at an ageing temperature of 950.degree. F. or higher. Also, when combined with the addition of small amounts of Mo, Cr, W and V, these types of steel alloys can form M.sub.2 C.sub.x type carbide precipitates to inhibit the microvoide nucleation so as to strengthen the steels. These M.sub.2 C.sub.x types of carbides, unlike the M.sub.3 C, M.sub.6 C or M.sub.23 C.sub.6, are more favorable carbide precipitates that increase the toughness and fatigue strength. These are the major factors that make the secondary hardening steels extremely attractive to those applications requiring high strength, high hardness, and high fatigue resistance. Examples of such steels are those commercially available Carpenter Aermet 100 and AF1410 steels, which have excellent mechanical properties.
However, the secondary hardening martensitic steel alloys require the addition of relatively high concentrations of carbon and other elements to increase the final mechanical properties. This makes this type of steel not suitable for the applications requiring both high mechanical strength and high saturation induction. Neither Carpenter Aermet 100 steel nor AF1410 steel is designed for the applications that also require greater magnetic saturation.
The addition of Co to the steel alloys is an effective way to increase the magnetic saturation of the steel alloys. Theoretically, the Fe--Co binary alloy gets to the highest saturation induction 24.2 kilogauss (KG), with 35 to 37% of cobalt. However, there is an inherent brittleness problem associated with the binary Fe--Co alloy.
Adding Ni into the Fe--Co matrix tends to decrease the total saturation magnetization. The presence of high concentrations of Ni may result in the formation of austenite, a Face-Center-Cubic (FCC) non-magnetic phase, during the heat treatment. However, it is also requried to lower the M.sub.3 temperature and to promote the formation of lath martensite, which is the key alloy element for getting good mechanical strength.
One of the drawbacks associated with the Fe--Co--Ni ternary alloy is that the Fe--Co--Ni solid solution can only be hardened mechanically, but not thermally. Severe mechanical cold working significantly degrades the magnetic properties as well as the stability of the alloy due to the resulting cold-work residual stresses. An annealing process is therefore required to restore both the magnetic properties and alloy workability, and consequently reduce the inherited tensile and yield strengths. Therefore, other alloy elements need to be added to the Fe--Co--Ni alloy to make it possible for the applications, which demand high mechanical strength.
It is an object of the invention to provide the optimal chemical composition of age-hardening steel to achieve the best possible combination of the magnetic saturation induction and the mechanical properties for the hammerspring applications.
It is another object of this invention to provide the optimal Ni concentration of the Fe--Co--Ni alloy steel and also to provide the requried optimal amounts of the hardening elements, such as C, Cr, Mo, W, and V, to the Fe--Co--Ni alloy matrix to increase mechanical strengths.
It is a further object of the invention to provide an age hardened steel alloy having high tensile, yield and fatigue strengths, coupled with good soft magnetic properties including high saturation induction.
It is another object of the invention to provide age hardened steels containing more than 20% by weight of Co and less than about 6% of Ni, which preferentially form low carbon martensite matrix or the mixtures of martensite and ferrite, rather than the simple ferrite phase, depending upon the solution heat treatment and ageing temperatures.
It is another object of the invention to provide a method for manufacture of age hardened steel alloys characterized by high tensile, yield and fatigue strengths and high saturation induction without any liquid-quench process.
It is another object of the invention to provide Fe--Co--Ni alloys having high tensile, yield and fatigue strengths and high saturation induction, which are particularly suited for use as hammerspring material for impact printers, and other applications where high saturation induction and high strength are desirable.
It is a final object of the invention to provide a printer hammerspring and a printer hammerbank formed of the alloys of the invention and a printer incorporating the hammerspring and hammerbank.